1. 16 – 2 Electric Force

2. Charles Coulomb – 1780’s experimented to determine the electric force.
Used a torsion balance which works by isolating the electric charge (using insulators)
converts the resultant force into torque
can be easily and accurately measured.  

3. worked by charging two pith balls, one fixed the other attached to a silver wire attached to a micrometer
configuration allows the electrical force to be determined by transferring the force into torque. 

4. Found that the force between 2 charged objects is:
inversely proportional to the square of the distance
directly proportional to the product of their charges

5. Coulomb’s Law
Expresses the force relationship for charged particles.
Magnitude of force between two charged objects
k = 8.99 x 109 N•m2/C2, Coulomb’s Constant
Q1 = Magnitude of charge 1
Q2 = Magnitude of charge 2
Direction of force depends on sign of charges. Q1 Q2 r

6. Sample Problem
Compare the electrical to the gravitational force in a hydrogen atom (between proton & electron)
Given:
q= 1.60 x C
mp = 1.67 x kg
me = 9.11 x kg
r= 5.29 x 10 –11 m
G = 6.67 x Nm2/kg2

8. In other words the electrical force is 1039 times stronger than the gravitational force!

9. The electric force is a vector!
Superposition of forces Fnet = F1 + F2 + …